1. Field of the Invention
The present invention relates to a small-sized liquid crystal display device particularly suitable for PDAs, car navigation systems, laptop personal computers, small-sized television sets and the like, and particularly to a liquid crystal display device in which a stable ground line is formed therein the display panel to eliminate noise interference.
2. Prior Art
In recent years, liquid crystal display devices have come to be widely used in various kinds of instruments such as PDAs, car navigation systems, laptop personal computers, small-sized television sets and the like, for example, particularly for being compact, light-weight, and low in electric consumption.
Such liquid crystal display devices normally comprise a liquid crystal display device in which an upper transparent glass substrate and a lower transparent glass substrate are superposed at a predetermined gap, such that pixel electrodes made of transparent conductive film and a surface on which orientation film or the like are laminated facing each other, both substrates being bonded by sealing material provided around the rim of both substrates and liquid crystal is filled between the substrates, and a polarizing plate is further installed outside the substrates, a liquid crystal drive circuit board arranged outside the liquid crystal display device for driving the same, a backlight arranged under the liquid crystal display device for supplying light thereto, a frame body for holding each member, a metal shield case (frame) that houses each member and the opening of the liquid crystal display window, and an electric connector for supplying power to the liquid crystal drive circuit board and the like from the outside.
Of these constituent parts, the electric connector is normally designed for attachment to the side of the frame. However, because the frame is made of metal that requires working, the process of attaching the connector to the side of the frame becomes rather complicated. In addition, because the connector protrudes from the outer portion of the frame, there have been cases where the connector breaks down or is otherwise damaged upon collision with an object or the like coming from the outside. Further, there are cases where the protruding connector is not only unaesthetic but also impairs the design of the liquid crystal display device.
Consequently, there is known a liquid crystal display in which the connector is not disposed on the metal frame, but where a liquid crystal filling portion is formed on one corner of a liquid crystal display board, a liquid crystal drive circuit board being arranged apart from the liquid crystal filling portion, a connector is mounted on an installation board for electronic parts adjacent to the liquid crystal filling portion, and a connector exposure hole is provided in the region of the outer frame opposite the connector (for an example, refer to claim 1, FIG. 3 and FIG. 14 of Japanese Laid-Open Patent Publication No. 6-265925).
Further, recently, the liquid crystal elements of the liquid crystal display device have been micro-fabricated, with a large number of various electronic parts densely mounted on the circuit board, such that wiring them has become extremely dense and complicated. Moreover, due to increasingly higher driving frequency of the liquid crystal display device, various kinds of noise have been generated therein, and together with noise from the outside or the like intruding the liquid crystal display device, it has become impossible to achieve stable display. As a countermeasure, a method has been proposed whereby a ground line is formed within the device to restrain or neutralize the impact of such noise.
The ground line is normally formed on the surface of the liquid crystal drive circuit board, and there is known in the prior art a method whereby a frame ground pad electrically connected to the ground line is connected by solder to the metallic shield case of the liquid crystal display device via a lead wire, and a method whereby a cut is provided on the metallic shield case and a piece of nail integrally provided on the shield case is directly affixed to the frame ground pad by solder to form the ground line (refer to the right column of page 2, the right column of page 3 and FIG. 1 of Japanese Laid-Open Patent Publication No. 6-265922).
On the other hand, small-sized liquid crystal display devices used in car navigation systems or the like, for example, have been equipped with various kinds of software to be highly functional. For example, some liquid crystal displays equipped with standard software are designed in such manner that other software can be installed thereinto, depending on the preference of the user.
For such purposes, however, the above-described liquid crystal display device is not suitable for use due to limited housing space and design restrictions, requiring various changes in circuit layout as well as circuit wirings on the circuit board, the shape of the board and frame and the like. Furthermore, the installation position of the electric connector and the ground line have also been altered to conform to these changes.
Such liquid crystal display devices are already in the market and publicly known. FIG. 5 shows the plane rear view of a liquid crystal display device for a car navigation system.
The liquid crystal display 1A comprises a liquid crystal module 10A, a liquid crystal drive circuit board 20A which is mounted on the back case of the liquid crystal module 10A, with a size measuring approximately one third the size of the module, an electric connector 22A which is mounted on the liquid crystal drive circuit board 20A, and a metal cover 30A, on which a cutout 33A has been cut out so as to expose the electric connector 22A residing on the liquid crystal drive circuit board 20A while covering the liquid crystal drive circuit board 20A. Electronic parts are then arranged to fit into the smallest liquid crystal drive circuit board 20A possible, the electric connector 22A being positioned slightly remote from the rim of the circuit board. Thus, the structure of the metal cover 30A for covering the liquid crystal drive circuit board 20A is widely cut out so as to expose the electric connector 22A. Note that reference numerals 14A to 14D denote fasteners for fixing cables or the like attached on the back case, while reference numeral 22B denotes an electric connector, and reference numerals 23A and 23B denote cables.
In the liquid crystal display 1A, the liquid crystal drive circuit board 20A is grounded by the metal cover 30A. However, since the liquid crystal module 10A itself is covered by a metal case 12A which is connected to the metal cover 30A, the ground line has been omitted since there is no need to provide special electric connection means to connect the liquid crystal drive circuit board 20A to the metal cover 30A. Alternatively, special working is applied to the area near the central portion of the metal cover 30A, even though there is a provision for connection means, for the purpose of achieving connection.
As a result, when the ground line was not formed, or, when the ground line was not sufficient, countermeasures to prevent external noise from intruding the display device or to suppress the generation of noise within the display device became inadequate, making it difficult to achieve stability of display quality, and further causing the generation of unnecessary radiant waves that bring about electromagnetic interference (EMI). Upon investigation of the source of the noise, it has been further discovered that even the application of special working to the location near the central portion of the metal cover 30A provided with connection means is not a sufficient countermeasure against noise interference. Additionally, metal working such as the above-mentioned cutout was troublesome, since it was applied approximately near the central portion of the metal cover 30A.